60% of maximum volume is a poor metric due to a complete ignorance of decibels, power and how the two are related together.
If 60% means 0.6 times some maximum number of decibels, then this is not scale invariant. For instance 60 dB versus a maximum of 100 dB ("60%") is not the same power ratio as 30 dB versus 50 dB (also "60%"). In the dB scale, we subtract to show ratios; we don't indicate ratios between decibels. For instance, 10% of the maximum volume (in terms of power) means 10 dB below.
If 60% just means 60% of the volume position between 0 and MAX, that is also hokey because it depends on the actual taper curve. Analog volume pots use various ad-hoc "audio taper" curves. Digital volumes tend to be better: the steps based on dB increments. The increments are not always the same across the volume control range.
The maximum volume varies between amplifiers and headphones. What does that even mean? Maximum level at what percentage THD? Many sound systems can go a notch louder, if you accept greater distortion, and some people listen to rock that way.
A sensible starting guideline might be, say, "at least 20 dB below the maximum output intensity which 'typical' headphones can reproduce without distortion". That is still hokey in a number of ways, an important one of which is that the user usually has no interface for dialing in a 20 dB drop; such markings are only found on the controls of some pro audio gear.